Life is tough these days at NASA, the space agency that can’t launch anyone into space.

It wrestles with basic questions: Where to go? How to get there? When? And for what purpose?

It killed a plan to return to the moon and now is building a jumbo rocket to go to … well, it’s unclear. Maybe to an asteroid: a rock to be named later.

NASA is betting that private companies will create a commercial taxi for flights to low Earth orbit. In the meantime, NASA astronauts ride on aging Russian rockets that look increasingly creaky. At any given moment, a few Americans are on the international space station, circling the planet every 90 minutes, nearly as anonymous as they are weightless.

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But even as NASA goes through this awkward transition in human space flight, the agency has one bright spot: science. NASA’s scientific missions — robotic probes, telescopes, satellites — are bringing Earth, the sun, the solar system and the universe into sharper focus.

Science at NASA is not without serious problems, a fact expected to be reflected in the Obama administration’s budget request Monday.

The James Webb Space Telescope, the successor to the Hubble, has gone far over budget and is still years from launch. The next Mars rover has also experienced cost overruns. As a result, planetary science, one of the divisions within NASA’s science directorate, will suffer a sharp cut under the new Obama budget, according to scientists familiar with the administration’s plans. Scientists expect that NASA will terminate its collaboration on two European-led robotic Mars missions scheduled for later this decade.

The question is: To what extent will future science missions be squeezed, delayed or terminated by the NASA budget crunch? What’s certain is that NASA has managed in recent years to launch a formidable fleet of scientific instruments.

NASA’s internal chart shows 86 missions, involving 96 spacecraft, either in service or preparation. That doesn’t include the two European Mars missions. It does include other international collaborations, and the extended operations of aging spacecraft that have completed their primary mission and are still blinking away.

One probe, New Horizons, is on its way to Pluto. Another, Messenger, has been orbiting Mercury since March. A lunar orbiter launched in 2009 has mapped the moon in unprecedented detail, and two more NASA spacecraft achieved lunar orbit six weeks ago on a mission to study the moon’s gravitational field and interior structure.

NASA’s Juno spacecraft blasted off in August on a five-year mission to Jupiter. The robotic probe Cassini continues to study Saturn, and in a week will make another close pass of the huge moon Titan.

Kepler, a space telescope launched in 2009, has found 61 planets by last count, with many more candidate planets yet to be confirmed. The longer Kepler observes a small patch of deep space, the more likely it is that it will detect a true Earth twin — a planet that’s both Earth-size and in a propitious orbit that puts it in a star’s “habitable zone.”

NASA is eager to see what happens on the morning of Aug. 6, when the $2.5 billion Mars Science Laboratory, launched in November, lands in a crater and dispatches a souped-up rover, Curiosity, to look for signs that Mars was once warm, wet and teeming with Martian life. The laboratory will land on Mars using a never-before-deployed technology called a sky crane.

“It’s going to be an incredible nail-biter during the descent and landing, but then we’re going to have this amazing rover,” said John Grunsfeld, the astronaut and astronomer who became famous for his work fixing the Hubble and who now heads NASA’s science directorate.

Meanwhile, the sun, which has been in the news lately after spitting massive amounts of plasma at Earth and inciting a spectacular round of Northern Lights, is being scrutinized by NASA’s Solar Dynamics Observatory. Saturday marked the second anniversary of its launch.

The observatory has made discoveries that help scientists understand the phenomenon of “space weather.” It has found, for example, that solar flares are longer-lasting than anyone previously knew, and has managed to detect them when they’re in an embryonic phase deep beneath the sun’s surface.

“What we are learning from this is pretty amazing. It’s staggering,” said Madhulika “Lika” Guhathakurta, who works on the solar observatory as the lead scientist with NASA’s “Living With a Star” project.

She added: “Doing science with robotic experiments compared with doing human space flight is a piece of cake.”

Grunsfeld said the science missions benefit from a competitive environment. There are many great ideas for scientific missions in space. The scientists compete fiercely for limited funds.

The science missions sometimes suffer from the same problem that bedevils human spaceflight: flat budgets that extend the timeline of a program, push many of the expenses into the future, and require standing armies of employees. That boosts a project’s ultimate cost.

But the science missions still enjoy political and popular support. They have specific goals, and often produce dramatic results, such as the awesome images obtained by the Hubble telescope during its two decades in space.

“The payoff from science missions is pretty clear — new knowledge — accompanied in some cases by pretty pictures that make it publicly accessible. The payoff from human spaceflight is not nearly as clear and still controversial,” said John Logsdon, professor emeritus at George Washington University’s Space Policy Institute.

Human space flight isn’t a process that can turn on a dime, as NASA’s recent strategic decisions have shown. Inertial forces are powerful. And because there’s so much money involved, there are multiple stakeholders, including politicians representing states with NASA centers and major aerospace contractors.

Under pressure from powerful senators, NASA will pour billions of dollars into the SLS, the “Space Launch System,” which would create a heavy-lift rocket capable of flying to the moon or a distant asteroid. Critics have dubbed it the Senate Launch System.

The current schedule indicates that the rocket wouldn’t be ready for its first flight — unmanned — until 2017, and then in 2021 it would have a second flight, this time with astronauts aboard.

“To say with a straight face we’re going to spend $20 billion between now and 2021 for two launches, you know, is hard for a disinterested observer to accept,” Logsdon said.

Whether that rocket will ever fly is unclear, Logsdon said. He said the program is “in multiple dimensions fragile.”

Mission uncertainty is a serious problem in the ambitious and difficult enterprise of space travel.

Grunsfeld, whose career has straddled the human exploration and science sides of NASA, offered a comparison to the pharaohs and their pyramids. When they made a decision to build a pyramid, he said, they didn’t revisit the decision every year.